Nitrate - a "Barometer" of Tank Health

for the week of 1/29/98

Not to be confused with nitrite (NO2-), the very toxic ion common in aquariums during the break-in period, nitrate (NO3-) is the final product of fish waste decomposition. It is not in itself very toxic (although it may stimulate algae growth), but if it is accumulating, it is safe to assume that other compounds, more dangerous but harder to test for, are accumulating as well.

A well maintained aquarium achieves a sort of balance between the waste being introduced (including fish waste, uneaten food, and any other decaying organic material) and the waste being removed by filtration and water changes. Rising levels of nitrate simply indicate that the balance has been lost and more waste is being produced than removed. The aquarist needs to re-instate balance, either on the input end by keeping fewer fish or feeding less, or on the outtake side by increasing filtration and/or water changes.

Testing for nitrate is not difficult, but there can be two areas of confusion. First, make sure it is nitrate, with an "a" and not nitrite, you are testing for. Nitrite is almost always near zero once the initial break-in cycle is over. Second, find out whether the nitrate kit you're using tests for nitrate-ion (usually represented as NO3-) or nitrate-nitrogen (NO3-N). These are merely two different ways of measuring the same thing, and the results from either method can be easily converted to the other. To convert a nitrate-nitrogen reading to nitrate-ion, multiply by 4.4.

For fish-only tanks, we like to keep the nitrate-ion level under 100 mg/l (or about 20 mg/l nitrate-nitrogen), but reef-keepers and those with heavily lit "natural" planted aquariums might aim for nitrate-ion levels under 25mg/l (approximately 5mg/l nitrate-nitrogen).